The present invention relates to a process and device for reducing the spectral line noise inside an aircraft, especially a rotating-wing aircraft, and in particular a helicopter.
More particularly, it applies to the reducing of the noise in the cockpit and/or in the passenger cabin of said aircraft.
It is known that, on a rotating-wing aircraft, the acoustic spectra defined in the domain lying between 20 Hz and 20 kHz pertain to the superposition of noises of different origins, which can be clustered into two different groups depending on their spectral characteristics, namely pure sounds or spectral line noises and broadband noises.
In a known manner, pure sounds or spectral line noises occur especially, as the case may be:
at characteristic frequencies of the kinematic chain of the aircraft;
at the frequencies of rotation of the blades of the rotors (main and rear) and at the harmonics of these frequencies;
at the frequencies of rotation of the blades of the compressors of the turbomotor sets; and/or
at the frequencies of rotation of the blades of the fans for cooling the main gearbox and/or of electrical equipment, as well as at the harmonics of these frequencies,
while broadband noises comprise especially as the case may be:
noise from a boundary layer developing on the fuselage;
noise generated by the rotors;
noise from the flow in the air inlets and nozzles;
engine noise; and/or
noise from the air conditioning or heating circuits of the cockpit or of the passenger cabin.
Although all these noises may pose problems, the acoustic annoyance experienced by the passengers and the crew is caused essentially by spectral line noises. Consequently, the main object of the present invention, which reckons to limit this acoustic annoyance, is to reduce said spectral line noises.
There are various known solutions for reducing such noises inside a rotating-wing aircraft, especially a helicopter.
A first known solution has the object of reducing the vibratory level of or the radiation from noise sources and/or the fuselage. For this purpose, various physical actions may be implemented, especially:
a reduction in the vibrations of the structure and/or mechanical members, by damping or modifying the stiffness or the mass;
an attenuation in the acoustic transmission, by damping or modifying the stiffness or mass;
a double-partition effect, by shrouding the relevant source;
acoustic absorption by fibrous or cellular materials; and
acoustic absorption by Helmhotz resonators.
The first four physical actions above make it possible to decrease the general level of noise in a wide domain of frequencies, but they entail a considerable and very disadvantageous increase in mass. Moreover, the noise decrease then obtained is not selective enough to dispose of the acoustic annoyance specific to pure sounds.
On the other hand, the fifth and last physical action above makes it possible effectively to reduce the spectral line noise, although only in a narrow band of frequencies, defined during the design.
This above first solution based on passive processing of the noise is therefore hardly effective, especially for spectral line noises generated by vibratory or acoustic excitations dependent in particular on the rotation regime, prone to variations as a function of time, of rotating machines.
A second known solution advocates active noise control.
The document WO-98/06089 discloses a system for actively controlling noise and vibrations in the cabin of an aircraft. This known system comprises:
sensors for measuring the values of vibratory and/or acoustic parameters representative of vibratory and/or acoustic effects of sources of noise of said aircraft;
reference sensors for measuring the values of reference parameters;
active and controllable vibration absorber means which are able to reduce the vibratory and/or acoustic effects of said sources of noise; and
a main computer determining, on the basis of the values measured by said sensors and said reference sensors, control commands for said vibration absorber means, with a view to reducing the noise and the vibrations inside said aircraft.
Another example of active control is described in the document FR-2 732 807 which discloses a personal process and a personal device for acoustic attenuation. This known document envisages an attenuator assembly which comprises sensors and loudspeakers arranged in proximity to the head of a passenger of an aircraft. These loudspeakers are intended to create counter-noises in order to attenuate the noises existing in proximity to the head of the passenger, by combining with these noises.
This attenuator assembly thus only allows individual and very localized attenuation of the noise in proximity to a passenger""s seat.
This known device has numerous drawbacks. In particular:
it requires a number of loudspeakers and of microphones which is proportional to the number of seats for which one wishes to carry out acoustic attenuation, this proving to be expensive, bulky and penalizing in terms of mass, in particular for large cabins;
a loss of available volume in the cabin occurs; and
it is necessary to adapt the existing seats, this of course being expensive.
Consequently, this known solution, which is based on active processing which is both localized and individual, is hardly satisfactory for reducing the spectral line noise in a rotating-wing aircraft furnished with a large cabin which may contain a plurality of pilots and passengers.
The document FR-2 769 396 from the applicant describes a device making it possible to remedy these drawbacks. This known device is a device of reduced mass, cost and bulk making it possible effectively to decrease, in a substantially global and nonindividual manner, the noise, and more particularly the spectral line noise, inside a rotating-wing aircraft. For this purpose, this known device comprises:
sensors for measuring the values of at least one vibratory and/or acoustic parameter representative of a vibratory and/or acoustic effect of at least one source of noise of said aircraft;
controllable mechanical means able to create a loading capable of reducing the vibratory and/or acoustic effect of said source of noise; and
a control unit, for controlling said mechanical means, as a function of the values measured by said sensors.
In a particular embodiment, this known device comprises P sensors Cp arranged at points Mp and capable of measuring one and the same vibratory and/or acoustic parameter, and Q mechanical means Aq able respectively to create loadings capable of reducing one and the same vibratory and/or acoustic effect, and the control unit carries out, repetitively, the following successive operations:
it calculates, for each of said P sensors Cp, a value P1p satisfying the relation:
P1p=P2p+xcexa3q(Tq,p.P3q), 
xe2x80x83q varying from 1 to Q,
in which:
P2p corresponds to the value of said vibratory and/or acoustic parameter which exists at the point Mp in the absence of action of said device and which depends on the value measured by the sensor Cp;
P3q is the value of said vibratory and/or acoustic parameter, due to the action of the mechanical means Aq and dependent on the control of said mechanical means Aq; and
Tq,p is a value for transferring between the value of said parameter existing at the level of the mechanical means Aq and the corresponding value existing at the point Mp;
it calculates the sum:
xcexa3p|P1p|2, 
xe2x80x83p varying from 1 to P; and
it minimizes the above sum so as to deduce therefrom the control commands for said Q mechanical means Aq, which are addressed to these latter.
Consequently, this known device which can be adapted to the flight conditions, by controlling for this purpose said mechanical means, carries out in particular global attenuation, that is to say at least over the set of P controlled sensors, and not individual attenuation like the device described in the document FR-2 732 807, and is therefore particularly effective.
The object of the present invention is to perfect the teaching provided by the document FR-2 769 396.
It relates to a process for reducing the spectral line noise inside an aircraft, especially a rotating-wing aircraft and in particular a helicopter, which process makes it possible to reduce in a particularly effective manner the most annoying spectral line noise(s), whilst being adaptable to any variation (in particular in frequency and in amplitude) of the source(s) of noise and to any desired modification of the noise reduction (especially relating to its localization).
To this end, said process according to which the following operations are carried out repetitively and automatically:
a) for each of the I points Mi, i varying form 1 to I, situated on said rotating-wing aircraft, the value P2mi of a vibratory and/or acoustic parameter which exists at said point Mi is measured, said vibratory and/or acoustic parameter being representative of a vibratory and/or acoustic effect of at least one so-called primary source of noise of said aircraft;
b) for each of said I points Mi, a value P1i corresponding to the relation:   P1i  =      P2i    +                  ∑                  j          =          1                          j          =          J                    ⁢              (                  Tj          ,                      i            ·            P3j                          )            
xe2x80x83is measured in real time, in which relation:
P2i is representative of the value P2mi measured in the absence of active control;
Tj,i is a value for transferring between the value of a vibratory and/or acoustic parameter existing at the level of a controllable mechanical element, forming a secondary source of noise, and the corresponding value existing at said point Mi; and
P3j is a value of said vibratory and/or acoustic parameter, due to the action of said mechanical element and dependent on the control of the latter; and
c) on the basis of the values P1i measured in real time for all the I points Mi, a sum S is calculated, satisfying the relation:   S  =            ∑              i        =        1                    i        =        I              ⁢                  "LeftBracketingBar"        P1i        "RightBracketingBar"            2      
d) said sum S is minimized so as to deduce therefrom control commands for said J mechanical elements; and
e) the control commands thus deduced are applied to said J mechanical elements, is noteworthy according to the invention in that, repetitively and automatically, in a preliminary step, the value Vr of at least one reference parameter which is correlated with the noise from said primary source of noise is measured and, on the basis of said measured value of the reference parameter, a reference signal is determined, and in step d), said sum S is minimized by carrying out filtering with respect to said reference signal determined in said preliminary step.
Thus, since said reference signal is correlated with the noise, it is representative (especially in frequency and in amplitude) of the noise existing in the aircraft and hence of the strongest and most annoying spectral lines so that the noise reduction implemented by the present invention is targeted by appropriate filtering at these most annoying spectral lines, thereby making it possible to increase the effectiveness of the noise reduction and consequently the comfort of the pilots and the passengers.
Moreover, advantageously, in said preliminary step, a plurality of R reference values Vr is measured and the reference signal SR is calculated on the basis of the relation:       SR    =                  ∑                  r          =          1                          r          =          R                    ⁢              Cr        ·        Vr              ,
the R values Cr representing coefficients. Thus, through an appropriate choice of said coefficients Cr, it is possible to obtain a reference signal SR which manifests all the frequencies whose magnitude one wishes to reduce, especially in the xe2x80x9cSIL4xe2x80x9d frequency domain and for the entire flight domain, the xe2x80x9cSIL4xe2x80x9d frequency domain being defined by the four octaves with central frequencies situated at 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz respectively.
Furthermore, advantageously, before control is adjusted and used, in step b) above, each value P2i is determined from the relation:
P2i=xcex1i.P2mi, 
in which xcex1i is a weighting coefficient, preferably lying between 0 and 1. This makes it possible to carry out a spatially xe2x80x9cdiscriminatedxe2x80x9d reduction in the noise, by greater or lesser weighting of the values measured at the various points Mi. This characteristic makes it possible to favor, during the noise reduction, certain zones, for example certain passenger places on the aircraft, with respect to others or certain particular frequencies occurring in the noise spectrum.
Moreover, according to the invention, the number J of mechanical elements (loudspeakers and/or mechanical actuators) is less than or equal to the number I of points Mi. Consequently, it is possible to reduce the noise in a number of zones, which is greater than the number of mechanical elements representing the secondary sources of noise, envisaged for this purpose, this of course being particularly advantageous, especially as compared with the known device disclosed by the aforesaid document FR-2 732 807.
Within the context of the present invention, the aforesaid preliminary step of the process in accordance with the invention can be implemented:
either simultaneously with steps a) and b), that is to say in the course of the flight;
or in a phase prior to said steps a) and b), especially in a mission preparation phase.
More precisely, it will be noted that the acquisition of the R reference signals Vr can be effected in two different ways:
either, these signals are measured before the implementation of active control and the R coefficients Cr are selected. During control, the reference signal SR is utilized, with the coefficients Cr frozen;
or, the R signals Vr are measured during control, these being adaptable during the mission, and the user has the possibility of then adjusting the coefficients Cr himself. During effective control, the utilization of the reference signal remains identical, of course, to the previous case.
It will be noted moreover that the measurements P2mi will have to be acquired during an identification flight, before the flights with control. This is because this involves adjusting the dynamic range behavior of the acquisition chain (sensors, recorders, etc.) so as to ensure a sufficient margin so as not to saturate the amplifiers, etc.
The present invention also relates to a device for reducing the spectral line noise inside a rotating-wing aircraft.
This device, which is of the type of that disclosed in the aforesaid document WO-98/06089, comprises in a known manner:
sensors for measuring the values of vibratory and/or acoustic parameters representative of at least one vibratory and/or acoustic effect of at least one primary source of noise of said rotating-wing aircraft;
at least one reference sensor for measuring the values of at least one reference parameter which is correlated with the noise of said rotating-wing aircraft;
controllable mechanical elements forming secondary sources of noise which are able to create, under the effect of control commands, loadings which are capable of reducing the vibratory and/or acoustic effect of said primary source of noise; and
a main computer determining, on the basis of the values measured by said sensors and said reference sensor, control commands for said mechanical elements, with a view to reducing the spectral line noise inside said aircraft.
According to the invention, this device is noteworthy in that said main computer determines said control commands by calculating a sum S satisfying the relation:   S  =            ∑              i        =        1                    i        =        I              ⁢                  "LeftBracketingBar"        P1i        "RightBracketingBar"            2      
on the basis of I values P1i measured by said sensors and by minimizing said sum S by means of filtering with respect to a reference signal which depends on the measured values of said reference parameter.
Thus, by virtue of the invention, said device makes it possible to implement noise reduction targeted at the most annoying spectral lines, as indicated hereinabove.
It will be noted that said auxiliary computer is not necessary when the reference signal corresponds to the value of a single reference parameter, which value is measured by a single reference sensor.
Moreover, by virtue of the invention:
one is able to adapt said noise reduction device to the flight conditions, by controlling the mechanical means;
by reason of a reduced number of components, the device in accordance with the invention is relatively compact, light and inexpensive;
since its components are independent of the aircraft, said device can be embodied in the form of an optional ready-to-mount control kit capable of being mounted on any type or rotating-wing aircraft; and
said device is capable of reducing all annoying spectral line noises liable to exist, regardless of their frequency, below 10 kHz and especially in the xe2x80x9cSIL4xe2x80x9d frequency domain.
Furthermore, said device advantageously comprises means for weighting the values measured by said sensors.
Moreover, according to the invention:
said sensors are microphones for measuring acoustic parameters and/or accelerometers for measuring vibratory parameters; and
said mechanical elements are loudspeakers and/or mechanical actuators of standard type.
The figures of the appended drawing will elucidate the manner in which the invention may be embodied. In these figures, identical references denote similar elements.